1. Field of the Invention
This invention relates to a copolymer which is particularly useful for making a positive resist.
2. Description of the Prior Art
The recent improvement of lithography, as well as processing techniques, is so remarkable that photolithography is making possible the resolution of images which are so close to each other as they have a distance of only 0.5 micron therebetween. Electron-beam lithography is used for making various kinds of custom devices, as it can do a finer or more precise work than photolithography, though it has a lower throughput.
Poly(butene-1 sulfone)(PBS) has a high sensitivity to electron-beam radiation and is widely used as a positive resist for making photomasks. Its scope of uses is, however, limited by its low dry etching resistance.
A multylayer resist process has recently been drawing attention as being applicable to fine lithography. Various attempts have been made to obtain polymers having oxygen-reactive ionic etching (RIE) resistance. Silicon-containing resists for photo and electron-beam patterning have been developed as one of the products of those attempts. Most of these resists are, however, negative, and only a few of them are positive resists having oxygen-reactive ionic etching (RIE) resistance.
Examples of the few positive resist materials are poly(3-butenyltrimethylsilane sulfone) as disclosed at the 7th International Conference on Photopolymers, New York, 1985, and poly(trimethylsilylstyrene sulfone) (PTMSS) as disclosed in Japanese Patent Application Laid-Open No. 215628/1987. Neither of these two kinds of polymers is, however, satisfactory. The former is low in heat resistance and is not satisfactory in oxygen-RIE resistance, either. The latter is not satisfactory in oxygen-RIE resistance, either, though its heat resistance is excellent.